1. Field of the Invention
The present invention relates to phase conjugation, and more specifically, it relates to a Stimulated Brillouin Scattering phase conjugate mirror constructed of all solid-state materials.
2. Description of Related Art
Phase conjugate mirrors employing Stimulated Brillouin Scattering (SBS) have become very useful and in some cases essential in high power laser systems. These mirrors, placed at the end of an amplifier chain somewhere in the mid-range of the amplification path, reflect the light with a phase wave front that is nearly exactly the inverse of that of the incoming laser beam. The reflected light retraces its path through the amplifiers, canceling out any wave front distortions that accumulated in the forward direction. This results in near aberration free output beams that exhibit the minimum beam divergence allowed by optical diffraction, hence the description "diffraction-limited."
The mechanism responsible for the reflectivity of the SBS mirrors is the generation of an intense acoustic wave inside the SBS nonlinear material. This acoustic wave serves as a very efficient Bragg grating which reflects the incoming light. Since the acoustic grating travels at the speed of sound through the material, in the same direction as the input light, the reflected light is frequency shifted from the input light by 100 Mhz to &gt;10 Ghz, depending on the SBS medium. The frequency shifted output is referred to as the Stokes wave and the frequency shift is referred to as the Stokes shift. The active material for the conjugators has most often been a liquid or high pressure gas. However, there is a strong incentive to design a Stimulated Brillouin Scattering (SBS) phase conjugate mirror that does not use a liquid or a high pressure gas as the nonlinear medium. The very high gas pressure (50-100 atm) required for gases such as Xe or N.sub.2 poses a potential hazard to the laser user and, its local environment and makes much more challenging the laser's successful operation over the large temperature ranges required for some operations in the field. The safety issues associated with the gas pressure also significantly increase the laser's cost. The two liquid SBS media which have been successfully demonstrated in working high average power solid state laser systems (carbon tetrachloride and Freon 113) are both banned for delivery for DoD applications because of their suspected role in ozone depletion. Their manufacture is scheduled to be discontinued in the very near future. Other useful liquids, such as the metal tetrachlorides (Ge, Ti, and Sn) are chemically unstable when exposed to the atmosphere, producing toxic and highly corrosive reaction products such as HCl.
As evidenced in "Wavefront Inversion in Induced Mandel'shtam-Brillouin Scattering in a Glass Without Failure," Sov. Phys. Tech. Phys. 25, 645-646 (1980), it has been recognized for many years that fused silica exhibits an SBS gain coefficient comparable in magnitude to that of many commonly used liquids and high pressure gases. When fused silica is used as the nonlinear medium in an SBS mirror, however, input pulses with energies above threshold for stimulated Stokes return often cause material breakdown and damage at the optical focus in the glass. This has sometimes been partially alleviated by using very long focal length lenses to decrease the beam irradiance at focus. This is not a good solution since, not only does this result in a physically large and unwieldy device, it also causes unacceptable temporal reshaping and shortening of the reflected pulse. In the above reference, breakdown was observed for all focal lengths of &lt;50 cm.
An optical architecture was proposed in "Methods for SBS Threshold Reduction," A. M. Scott, et al., Naval Research Laboratory, NRL/FR/5640--94-9704. that uses three foci in a liquid SBS gain medium, with the first and third physically overlapping, to reduce the energy threshold of an SBS mirror. However, the present inventors have recognized that the SBS enhanced four-wave-mixing process of this architecture has a far more profound effect on the performance of an SBS phase conjugate mirror than to merely reduce the energy threshold. By providing a feedback path for the Stokes light, the output of the SBS loop architecture is not dependent on noise generated at optical focus to sustain it. This provides very high temporal stability, eliminating potential phase drifts or jumps in the reflected light. It is now understood that these instabilities are the source of the damage that has been observed in previous experience with solid state SBS gain media. The SBS nonlinear gain coefficient for fused silica is larger than that for competing nonlinear processes such as self-focusing and multi-photon absorption. For this reason, the SBS process might be expected to dominate the light interaction in the medium, shielding the optical focus from an irradiance high enough for the competing processes to reach threshold. However, this is not the case in the presence of temporal instabilities during the pulse. The transiency introduced by these instabilities allows the competing processes to reach threshold and generate optical breakdown, damaging the glass.
FIGS. 1 and 2 show prior art optical configurations of solid-state SBS phase conjugate mirrors. FIG. 1 shows a conventional configuration in which an incoming laser pulse 10 is simply focused into the SBS medium 12 with a single lens 14. FIG. 2 shows the configuration described in the Scott, et al. reference described above in which two of three foci overlap to form a four-wave-mixing resonance. A laser beam 20 is focused into the SBS medium 22 with lens mirror 24. After passing through a focus at 26, diverging beam 20 is collimated by lens 28 and reflected by a high reflector (HR) 30 back into the SBS medium 26 and brought to a focus at 32 by lens 28. Lenses 24 and 28 together comprise a confocal lens pair. After passing through focus 32, beam 20 diverges and is collimated by lens 24. This collimated beam is reflected by high reflector 34 through lens 24, focus 26, and lens 28 to exit the system.
The optimal configuration of a multi-pass, multi-resonant optical architecture can completely eliminate temporal phase instability during the stimulated scattering process and thus prevent any damage to the solid state SBS medium. Since short focal length lenses can now be used, the result is a very compact and physically rugged phase conjugate mirror which requires no liquids or high pressure gasses.